CO7 does not (yet) replace all the functionality of Media Pro.
Media Pro is very strong in cataloging and handling metadata for large collections of images. It also recognizes more file formats than CO7. It offloads image processing to other programs so it interacts pretty well with PS as well as CO7 (and probably Aperture - but I don't use Aperture)
Also - Media Pro catalogs are very cheap. It's easy to create one for any collection of images, mess with the metadata or whatever and then either save it or blow it away. CO7 can create Media Pro catalogs (as well as import them) so there is a reasonably good interoperability between the two programs.
CO7's catalogs can actually contain your images - on a Mac catalog actually looks similar to a Mac application, so you can 'inspect' it by using Finder -> Show Contents. Media Pro does not do this: Media Pro catalogs are a database file which can live anywhere and is just used for managing the assets.
And - Media Pro documentation is somewhat opaque.
The program has been through several owners, name changes and multiple revisions.
I use both CO7 because I find their strengths complement each other and because CO7 catalogs are unstable on my system (have no idea if this is systemic or just my problem). My current workflow is to use CO7 sessions, manage some of the metadata in CO7 and the rest in Media Pro. This requires telling Media Pro to import and export annotations in order to keep the XMP files up to date (Media Pro keeps the meta data in its catalog otherwise. CO7 can be told to keep an eye on the XMPs). This seems to work well to keep Media Pro, CO7 & Adobe products in sync.
Regarding your tiff problem: no idea. CO7 presumes it's working with raw camera data and does all of its adjustments as instructions in .cos (I think) files. I don't know what Aperture does, but PS mucks with the pixels destructively. If your tiff has been 'adjusted' then this might be the source of the problems you're seeing.